Image tube with truncated conical anode and a plurality of coaxial shield electrodes



Oct.

STOUDENHEIMER ET AL 3,280,356

IMAGE TUBE WITH TRUNCATED CONICAL ANODE AND A PLURALITY OF COAXIALSHIELD ELECTRODES Filed July 17, 1958 a v y L 20 -22 F527 1 34 I Q I 33i 60 I l J ch: 33 48 =2; 3r

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WLZZM a M TIOIPNEY United States Patent INIAGE TUBE WITI-l TRUNCATEDCONICAL ANODE AND A PLURALITY 0F COAXIAL SHIELD ELECTRODES Richard G.Stoudenheimer, Lancaster, and Joseph C. Moor, Manheim, Pa., assignors toRadio Corporation of America Filed July 17, 1958, Ser. No. 751,591 5Claims. (Cl. 313-65) This invention relates to tubes of the type whichproduce a visible image in response to radiant energy, such asultra-violet light, visible light, infrared rays or X-rays. Inparticular, this invention relates to tubes of this type, which arenormally referred to as image tubes, or to image sections of morecomplex tubes or of multiple section tubes in which it is desirable toproduce a bright, clear image in response to a relatively weak inputsignal image.

An image tube, or an image section of a more complex device, is an imagedevice in which there is at least one difference of potential betweentwo electrodes. One of these electrodes is a negative electrodeincluding an electron-emissive photocathode and the other is a positiveelectrode including an anode and a phosphor screen. Image devices areknown in the prior art but these devices are so constructed that theycannot be operated at desirably high electrical potentials. Also, thesetubes tend to produce inferior, distorted images on the phosphor screen.In addition, these devices are constructed so that they frequentlysuffer from field emission if they are operated at relatively highelectrical potentials. Field emission is the emission of electrons fromone surface that has a high electric field applied between that surfaceand another surface. Any field emission which bombards the phosphorscreen in devices of this type results in a background image, or noise,which tends to obscure faint signal images.

The objects of the invention are to provide an improved image device, orimage section, that is capable of operating at a comparatively highdifference of potential between its two electrodes and one that iscapable of producing high quality visible images without suffering fromfield emission or image distortion.

An image device according to this invention includes a sphericallycurved photocathode adapted to produce an electron image correspondingto a light, or other radiant energy, image and a truncated conical anodeelectrode which is arranged substantially coaxial with the photocathode.Such an arrangement tends to provide spherical, equipotential surfacesbetween these electrodes which promote optimum focusing of an electronimage onto a phosphor screen positioned substantially at the base orlarger diameter of the truncated conical anode. An electrical shieldingmeans, in the form of generally tubular electrodes, is electricallyconnected to the photocathode and surrounds the space between thephotocathode and the truncated conical anode. The smaller diameter endof the truncated conical anode is provided with a planar cap, thediameter of which controls the convergence of the electrical fields nearthe anode and thereby controls the focusing action of the tube. The endof the shield that is remote from the photocathode is embedded in aglass wall portion of the envelope, and the portion of the shield thatis closely adjacent to the 'ice glass wall is coated with a thin film ofglass or feathered edge. This thin glass coating over metal allowselectrical charges on the glass to gradually leak through the thin glassto the metal whence they are dissipated. Thus, any build-up of charge onthe glass, and resultant field emission, is prevented. The variouselectrodes of the tube are shaped to promote the production of adistortion-free image and an optimum mounting of the materials which areused in preparing the photocathode and in gettering the tube.

The invention is described in greater detail by reference to the singlesheet of drawings wherein:

FIG. 1 is a sectional elevational view of an image tube embodying theinvention; and

FIG. 2 is a sectional view along the line 22 in FIG. 1.

An image tube 10, according to this invention, includes an envelope 12which is generally tubular in form and has, at one end, a metal ring 14which has a spherical glass face plate 16 sealed across the open portionof the ring 14 and transverse to the longitudinal axis of the envelope12. A photocathode 18 is formed on the inner spherical surface of theface plate 16. The radius of curvature of the photocathode 18 should besuch that the center of the radius of curvature is approximately betweenthe object distance and twice the object distance, for minimum imagedistortion, so that the electron image will travel most of its path inthe direction of a radial field toward the anode. The photocathode 18may be of any suitable type, for example, antimony cesium,silver-bismuth or the like. The photocathode 18 may be made responsiveto any desired wave length, such as visible, infrared, X-rays etc., bythe selection of known materials that are sensitive to the desired wavelengths. The ring 14 carrying the face plate 16 is sealed to one end ofa hollow metal cylinder 19, which forms a portion of the envelope andwhich forms a portion of the cathode assembly. The other end of themetal cylinder 19 is turned inwardly into a smoothly formed U-shapedannular portion 20 to avoid any sharp corners which might hinder theelectrical operation of the tube by field emission.

The envelope 12 further comprises an open-ended hollow glass cylinder 22which is sealed at one end to the U-shaped portion 20 of the metalcylinder 19 with which it is axially aligned. The other end of the glasscylinder 22 is provided with first and second concentric metal rings 24and 26, respectively. The metal rings 24 and 26 are sealed together attheir peripheries. The second ring 26 has a fiat glass plate 28 sealedacross it and transverse to the axis of the envelope 12. The innersurface of the glass plate 28 is provided with a suitable phosphorscreen 30 which acts as an electron collector. The phosphor screen 30 isaluminized and the materials therefor may be selected from any of theknown electron sensitive phosphors, one example of which iszinc-cadmium-sulphide activated with silver. An exhaust tubulation 31 isprovided in the wall of the glass cylinder 22. Three metal pins 33 aresealed through the glass portion of the envelope 12 and are spaced apartapproximately Electrodes mounted inside the envelope 12 include acup-shaped hollow cylindrical electrode 32 which is secured at one endto the U-shaped portion 20 of the metal cylinder 19. The upper end ofthe cylindrical electrode 32 gradually curves over the end of theU-shaped portion 20 so that no sharp corners are present, again toprevent field emission. The electrode 32 has a straight side wall 34which terminates in an inwardly turned end plate 36 having a large areaaperture 38 therein. The side Wall 34 is substantially parallel to butspaced from the glass cylinder 22. The cup-shaped electrode 32 forms anelectrical extension of the metal cylinder 19 and shields the signalelectrons from any charges which may accumulate on the glass portion ofthe envelope 10. Also, the cupshaped electrode 32 shields the glasscylinder 22 from the evaporated materials which are used to form thephotocathode 18.

A hollow, substantially cylindrical, anode shield 40 is provided withinthe tube 10. The anode shield 40 has a first small open end 42 fromwhich its side wall 44 flares outwardly to a second larger open end 46.The minimum diameter of the shield 40 is within the approximate range of0.5 to 0.75 the maximum inside diameter of the envelope 12 near thephotocathode 18 in order to produce the proper electrical fields. Thefirst open end of the anode shield extends through the aperture 38 andis positioned within the cup-shaped electrode 32. Thus, the highelectric field that is in the region around the anode does not reach thesharp edges of the aperture 38. In accordance with a feature of thisinvention, the large open end 46 of the anode shield 40 is embedded inthe glass cylinder 22 with the rim of the end 46 covered with athickness of glass 47 which insulates this end of the shield from theoutside of the envelope. It has been found that, if the end of the anodeshield 40 were extended through the glass tube 22 to the outside of theenvelope 12, an electrical leakage path would be provided between thisexposed end and the anode ring 24. If this leak-age path, along withcorona discharges, were present, the maximum voltage at which the tubemight otherwise be operated would be substantially reduced.

A thin film, or feathered edge, 48 of glass is also provided on theinner and outer walls of the cylindrical shield 40 within the envelopeand adjacent to the end 46. The film 48 of glass may be of the order ofto 15 mils thick and may be formed by applying a glass frit prior tomaking the seal between the electrode 40 and the envelope wall. Thisfilm of glass prevents field emission from the shield 40 since it hassufficient electrical conductivity so that an electrical charge built upon the glass wall will leak off through the electrode, which is groundedduring operation of the tube. The anode shield 40 is electricallyconnected to the cup-shaped electrode 32 by means of a wire 49. Thusfar, the cathode or negative electrode assembly has been described. Itcan be seen that the cathode assembly comprises a stepped cathodecylinder made up of axially aligned cylinders 19, 32 and 40 which becomeprogressively smaller in diameter the more remote they are from thephotocathode 18.

The positive electrode portion of the tube includes a truncated conicalanode 50 that is axially aligned with the cathode cylinders, and has asmaller diameter 52 and a larger diameter, or base, 54. The smallerdiameter of truncated conical anode is disposed substantially at thecross over point for the electronsemitted by the photocathode. Thus,during operation of the device the object distance is the axial spacingbetween the photocathode 18 and the smaller diameter of the truncatedconical anode While the image distance is the axial distance between thesmaller diameter of the anode and the phosphor screen. Depending uponthe type of operation desired, the image distance is selected to beWithin the approximate range of equal to the object distance to fivetimes the object distance. Furthermore, the small diameter 52 is withinthe shield 40 adjacent to the first small open end 42 thereof. The base54 of the anode 50 is secured to the first ring 24 and is disposedadjacent to the phosphor screen 30 that is on the glass plate 28. Thesmaller diameter of the anode 50 is provided with an apertured flat endplate, or focus disc, 56 having an outer diameter that is smaller thanthe inner diameter of the first small open end 42 of the shield 40. Thelongitudinal position of the end plate 56 is approximately the sameaxial distance from the photocathode 18 as the minimum diameter of theshield 40 in order to minimize image distortion and astigmatism. Theinner diameter of the end plate 56 is substantially the same size as theinner diameter of the smaller end of the truncated conical anode 50. Thefiat plate 56 may be in the form of a separate cap or it may be integralwith the smaller end of the andoe 50. The relationship between the outerdiameter of the plate 56, at the smaller end of the truncated conicalanode, and the inner diameter of the adjacent open end of the shield 40,controls the convergence of the electric fields near the anode and theresultant focus of the electron image, which is an inverted image, atthe phosphor screen. Astigmatism will occur in the reproduced image ifthe focus disc, 56, is out of round or misaligned with respect to theshield 40. The proper selection of diameters depends on the overalllength of the tube and the spacing between the photocathode and anode.Thus, for high magnification the outer diameter of plate 56 approachesthe inner diameter of the shield 40. For example, for a magnification ofapproximately 2.5 the outer diameter of plate 56 is approximately 0.8the size of the inner diameter of shield 40, and for a magnification of0.5 the outer diameter of plate 56 is approximately 0.3 the innerdiameter of the shield 40. It should be noted that the cathode cylinder,made up of electrodes 19, 32 and 40, completely shields the path of thephoto electrons between the photocathode and the anode 50.

The substances from which the photocathode is prepared may be includedin the tube as follows. One substance, for example silver, may bemounted as a pellet 58 on the wire 49 which is a heater wire and whichis connected between the shield 40* and the cup electrode 30. A pair oftroughs 60, see FIG. 2, are preferably mounted around the outer wall ofthe shield 42 and substantially midway between its ends. Electricalcontact is made to these troughs by means of pins or rods 33 sealed inthe glass tube 22. Cesium and barium emitting mixtures or the like maybe placed within the troughs 60. One advantage of this arrangement ofparts is that the cesium does not strike the photocathode surface orface plate directly, whereby non-uniformities in the photocathode mightresult. Another advantage of this structure is that, when using theindirect means for depositing the photocathode 18, the amount ofmaterial deposited may be more accurately and easily controlled. Thechemicals, as they are evaporated, are shielded by electrode parts andpass gently through the annular space 38. When evaporated in this way,the materials coat the photocathode uniformly.

The image tube 10 described herein is designed so that it operates infocus over a Wide range of voltages. In operation, the cathode isoperated at ground potential and the anode may be operated at a voltageas high as 20,000 volts. An example of typical operation poten tials isshown in FIG. 1. The combination of the stepped cathode cylinder and thetruncated conical anode provides favorably spherical equipotentialsurfaces between the photocathode and the smaller diameter end of theanode. Since field emission is substantially prevented within the tube,and since the electron flow path is completely shielded by electrodestructures, a high quality, distortion-free, image having good edgeresolution is provided on the phosphor screen of the tube.

In a typical construction, the tube 10 has the following dimensions ininches:

Overall length oftube 10 2.95 Photocathode 18 to screen 28 2.65,

Diameter 1.88 Photocathode 18 diameter 1.0 Length of tube 19 .875Diameter of tube 19 1.88 Length of tube 32 .325 Diameter of tube 32 1.0Length of tube 40 .416

Small diameter of tube 40 .7 Large diameter of tube 40 1.1 Photocathodeto tube 40 .964 Photocathode to anode 50 1.024

Anode cap 56 diameter .45 Anode smaller diameter .175 Anode length 1.0Anode base diameter .83 Anode smaller end to screen 1.463 Pins to ring1.5

The tube as described has been directed to a novel image tube. It shouldbe understood that it is within the contemplation of this invention touse a plurality of these structures, each functioning as a differentstage, to provide a multiple section or cascaded image tube. When thisis done, the light from the phosphor 30 of a first stage strikes thephotoemissive member, similar to member 18, of a second stage. When thisis done, the photoemitter for the second stage may be deposited directlyonto the exposed surface of the end 28. Also, when a cascaded multiplesection structure is desired the glass plate 28 is made as thin aspossible to prevent image distortion between sections, since it need notsupport a vacuum. The glass plate 28 is curved in a multiple sectiondevice to match the curvature of the photocathode glass 16 as shown inFIG. 1.

Furthermore, it is within the contemplation of this invention to usethis invention as an image section of other types of tubes such as theimage section of an image intensifier image orthicon type pickup tube.

What is claimed is:

1. An image tube comprising a photocathode, a plurality of electricallyconnected hollow cylindrical members decreasing in diameter from a largeopen end adjacent to said p-hotocathode to a small open end, saidphotocathode being disposed transverse to the axis of said hollowcylindrical members and substantially at said large open end thereof,the last two of said plurality of hollow cylindrical members from saidlarge open end being nested and having a small annular space betweenthem, a chemical carrier mounted on the outer wall of the last of saidcylindrical members and on the side of said annular space remote fromsaid photocathode, a truncated conical anode having a smaller diameterand a base axially aligned with said cylindrical members, said smallerdiameter of said anode being disposed within the last of saidcylindrical members and substantially at said small open end of saidcylindrical members, the crossover point for electrons from saidphotocathode being within said smaller diameter of said anode, anopenended hollow glass cylinder coaxially positioned with respect tosaid cylindrical members and sealed to the first of said cylindricalmembers and spaced laterally from the remainder of said cylindricalmembers, said glass cylinder terminating adjacent to the base of saidanode and closed by a collector electrode disposed in the electron pathdefined by said photocathode and anode.

2. An image tube including a hollow cylindrical envelope having aspherical glass member closing one end thereof, a photoemissive layerformed on said glass member, a hollow cylindrical electrode comprising aportion of said envelope in contact with said glass member and extendinglongitudinally therefrom, a hollow glass cylinder forming-the remainderof said envelope and extending from said cylindrical electrode, a hollowtruncated conical electrode positioned within said envelope and securedto said glass cylinder, a metallic cylindrical shield extending fromsaid cylindrical electrode and lying between said conical electrode andsaid glass cylinder and terminating in said glass wall, a portion ofsaid shield closest to said conical electrode being coated with a thinfilm of glass, and a collector electrode closing the other end of saidenvelope.

3. An image tube including a hollow cylindrical envelope having aspherical glass member closing one end thereof, a photoemissive layerformed on said glass member, a hollow cylindrical electrode comprising aportion of said envelope in contact with said glass member, a hollowglass cylinder forming the remainder of the envelope and extending fromsaid cylindrical electrode, a hollow truncated conic-a1 electrodepositioned within said envelope and supported by means including saidglass cylinder, a two-part metallic cylindrical shield extending fromsaid cylindrical electrode and lying between said conical electrode andsaid glass cylinder and terminating in the glass wall, the two parts ofsaid shield having a small annular space between them, the portion ofsaid shield which terminates in said glass wall being coated with a thinfilm of glass, and a collector electrode closing the other end of saidenvelope.

4. An image tube including an envelope, said envelope having a firstmetal wall portion and a second glass wall portion, said portions beingcoaxial and having adjacent ends and remote ends, the adjacent endsbeing sealed together, a face pate sealed to a sealing ring, saidsealing ring being sealed to said remote end of said metal wall portionand supporting an electron-emissive photocathode, a glass plate-likemember sealed across said remote end of said glass wall portion andsupporting an aluminized fluorescent screen, a two-part stepped tubularelectrode having one end sealed to the adjacent end of said metal wallportion and its other end embedded in said glass wall portion, agenerally hollow truncated conical anode having its smaller diameterdisposed within said two-part electrode and a base disposed adjacent tosaid screen, the cross-over point for electrons coming from saidphotooathode to said electron collect-or being within said anode andsubstantially at said smaller diameter.

5. An image device comprising an elongated envelope having two ends, aspherical shaped transparent platelike member closing one of said endsof said envelope, a photoemissive member on the inner surface of saidtransparent member, a second transparent plate-like member closing theother of said ends of said envelope, a phosphor screen on said secondplate-like member, a hollow truncated conical anode extending fromadjacent said second plate-like mem ber toward said spherical plate-likemember and terminating in a smaller end in spaced relation from saidspherical plate-like member, the center of curvature of said sphericalplate-like member being spaced from said spherical plate-like member bya distance within the range of the spacing from said sphericalplate-like member to said smaller end and twice the spacing from saidspherical plate-like member to said smaller end, a multiple sectionshield extending from said photocathode toward said anode andterminating around said smaller end, the minimum diameter of said shieldbeing within the range of one half to three fourths the maximum insidediameter of said envelope adjacent to said spherical platelilre member,the spacing between said smaller end and said second plate-like memberbeing within the approximate range of equal to and five times thespacing between said smaller end and said spherical plate-like member.

References Cited by the Examiner UNITED STATES PATENTS 2,402,602 6/1946Cheuigny 313246 2,411,523 11/1946 Cheuigny et al 313246 (Otherreferences on following page) 7 UNITED STATES PATENTS Vlach 313246Longini 313-65 Bailin 313107 Ruedy 250213 Carlo 313-65 Fries 250213Fries 250-213 8 2,851,625 9/1958 Ruedy et a1. 313-65 2,857,589 10/1958Pong 250-213 2,928,969 3/ 1960 Schneeberger 313-65 RALPH G. NILSON,Primary Examiner.

CHESTER L. IUSTUS, FREDERICK M. STRADER,

MAYNARD R. WILBUR, Examiners.

D. G. BREKKE, M. A. LEAVITI, Assistant Examiners.

1. AN IMAGE TUBE COMPRISING A PHOTOCATHODE, A PLURALITY OF ELECTRICALLYCONNECTED HOLLOW CYLINDRICAL MEMBERS DECREASING IN DIAMETER FROM A LARGEOPEN END ADJACENT TO SAID PHOTOCATHODE TO A SMALL OPEN END, SAIDPHOTOCATHODE BEING DISPOSED TRANSVERSE TO THE AXIS OF SAID HOLLOWCYLINDRICAL MEMBERS AND SUBSTANTIALLY AT SAID LARGE OPEN END THEREOF,THE LAST TWO OF SAID PLURALITY OF HOLLOW CYLINDRICAL MEMBERS FROM SAIDLARGE OPEN END BEING NESTED AND HAVING A SMALL ANNULAR SPACE BETWEENTHEM, A CHEMICAL CARRIER MOUNTED ON THE OUTER WALL OF THE LAST OF SAIDCYLINDRICAL MEMBERS AND ON THE SIDE OF SAID ANNULAR SPACE REMOTE FROMSAID PHOTOCATHODE, A TRUNCATED CONICAL ANODE HAVING A SMALLER DIAMETERAND A BASE AXIALLY ALIGNED WITH SAID CYLINDRICAL MEMBERS, SAID SMALLERDIAMETER OF SAID ANODE BEING DISPOSED WITHIN THE LAST OF SAIDCYLINDRICAL MEMBERS AND SUBSTANTIALLY AT SAID SMALL OPEN END OF SAIDCYLINDRICAL MEMBERS, THE CROSSOVER POINT FOR ELECTRONS FROM SAIDPHOTOCATHODE BEING WITHIN SAID SMALLER DIAMETER OF SAID ANODE, ANOPENENDED HOLLOW GLASS CYLINDER COAXIALLY POSITIONED WITH RESPECT TOSAID CYLINDRICAL MEMBERS AND SEALED TO THE FIRST OF SAID CYLINDRICALMEMBERS AND SPACED LATERALLY FROM THE REMAINDER OF SAID CYLINDRICALMEMBERS, SAID GLASS CYLINDER TERMINATING ADJACENT TO THE BASE OF SAIDANODE AND CLOSED BY A COLLECTOR ELECTRODE DISPOSED IN THE ELECTRON PATHDEFINED BY SAID PHOTOCATHODE AND ANODE.